I know this sounds weird for a 2018, but it sounds like you have the wrong clutch cable on there, if removing it stops the slipping. @Duckster hit it right on, not enough bare cable sticking out the ends of the cable sleeve.

I see the scrape marks on the clutch lever and mirror, you said (in the duplicate thread) that your friend dropped the bike. Did this friend do more damage than admitted to? and grab a replacement cable out of the box of spare parts for dirt bikes & lawn mowers? or has it always been like this and the cause of the friend dropping it? and perhaps the PO might have swapped in any handy cable from their collection of parts? or did you order a replacment one to fix the damage from your friend and possibly received a cable for a 250 instead of for a 300?

I can imaging a lot of scenarios, but I'll bet tomorrow's lunch money a new and correct cable will fix your problem.